docs: def get_flag_variables(ds):
      '''
      Returns a list of variables that are defined as flag variables
  
      :param netCDF4.Dataset ds: An open netCDF4 Dataset
      '''
      flag_variables = []
      for name, ncvar in ds.variables.items():
          standard_name = getattr(ncvar, 'standard_name', None)
          if isinstance(standard_name, basestring) and 'status_flag' in standard_name:
              flag_variables.append(name)
          elif hasattr(ncvar, 'flag_meanings'):
              flag_variables.append(name)
      return flag_variables